A typical line scan camera comprises a series of discrete optical sensors arranged in a line to form a linear sensor array. An optical arrangement, also forming part of the line scan camera, allows the linear array of optical sensors to form a one dimensional image of a line portion of the object. By scanning the camera relative to the object, so that the line portion moves across the object, a two dimensional image of the object is constructed.
One familiar application of a line scan camera is in the field of scanners. A flat bed scanner, for example, scans a sheet of paper and converts print on the sheet into a digitised image for transmitting to a computer. In this particular application, the object distance is specified by the structure of the scanner since the paper is assumed to be substantially planar and positioned on the "lat bed". Known flat bed scanners take advantage of this feature and employ optical arrangements consisting of a linear array of self focussing (SELFOC.TM.) lenses which relay the content of the paper at a known distance onto a linear array of image sensors. These self focussing lenses are each formed from a short section of optical fibre which are designed to operate at a relatively short, fixed working distance from the object. The self focussing lenses have the advantage of being relatively cheap to manufacture.
This invention relates to the application of a line scan camera in the field of circuit board inspection systems. The aim of these systems is to produce a two dimensional image of the inspected circuit board as viewed directly from above the circuit board, i.e., a plan view. This plan view is then examined to establish if the circuit board has been subject to any manufacturing process faults. Common faults include:
Misaligned parts PA1 Missing components PA1 Excess solder and solder bridges PA1 Rotated and reversed polarity parts PA1 Wrong component size PA1 Component leads that are lifted or bent
A first requirement of these systems is the ability to image three dimensional components mounted on the circuit board surface. As such, the optical arrangement used in these systems is characterised by having a relatively large depth of field. This allows objects raised from the substrate surface and objects on the substrate surface to be imaged in focus during a single scanning operation. A second requirement of these systems is the ability to produce a spatially accurate image of the circuit board to enable repeatable inspection results. As such, the optical arrangement used in these systems is also optimised to minimise imaging errors resulting from geometrical distortions such as lens field curvature, perspective distortion and parallax errors.
FIG. 1 shows a basic layout of an optical arrangement for a circuit board inspection system which is designed to have a relatively large scanning area together with a relatively large depth of field. The apparatus 10 comprises a conventional imaging lens 11, an aperture 12, and an image sensor array 13. The apparatus 10 scans across the substrate or circuit board 15 in the direction of arrow B, imaging a moving line segment A--A. A drawback with this arrangement is that at the ends of the line segment A--A, the lines of sight, represented by the rays R in FIG. 1, are not perpendicular to the substrate surface. This results in parallax errors. FIG. 2 illustrates in enlarged detail such a parallax error occurring in the end region of the line segment A--A. The ray labelled R is a line of sight to the optical arrangement 10, and is incident at an angle .alpha.to the plane of the substrate surface. Because the line of sight is riot perpendicular to the substrate surface, the relatively tall component 22 causes the hashed region S to be in shadow. Consequently, a portion of a neighbouring component 24 is obscured so that imaging information of this portion cannot be captured by the inspection system.
The inspection system addresses the problem of parallax by using telecentric lenses in place of the conventional imaging lens 11 and by limiting the scanning area. Unfortunately, this increases the cost of the system and slows down scanning time of the complete circuit board which is undesirable.